Elementary Reactions / Flames / RadicalsLarger hydrocarbon radicals in flames, which are difficult to detect by optical methods, can be analyzed quantitatively by radical scavenging with dimethyl disulfide in the condensed phase. -Gas samples taken from a sooting ethyne/oxygen flame by a nozzle beam were frozen at liquid-Ny-tcmpcraturc on the inner surface of a hollow sphere simultaneously with the radical scavenger dimethyl disulfide. The latter was admitted in large excess as a gas and was condensed uniformly on the inner wall of the sphere. The mixture of condensable stable flame products and scavenging products in the excess CH JSSCH 3 was separated and identified by GC/MS. -Larger hydrocarbons (C,HI' x~6) condensed quantitatively. H, CH 3 , C 4H 3 and the phenyl radical were scavenged as mono(methylthio) compounds; C 2 and the carbenes CH 2 , C 3H 2 , C;H 2 yielded bis(methylthio) compounds; other bis(methylthio) compounds observed were C 2nH 2(SCH 3 h with n = 1, 2, 3 and 4. CH, C 3H and C 2H3 were identified by the respective tris(methylthio) compounds. Any consecutive reactions of radicals with stable flame products and their recombinations in the cold trap were suppressed by their scavenging reaction.

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The method of scavenging free radicals sampled through a rarefied free‐jet from stationary low‐pressure high‐temperature reaction systems (flames, pyrolysis) in the condensed phase of dimethyl disulfide (DMDS) has been studied systematically. For this purpose the flat open condensing device originally used has been replaced by a condensation trap consisting of two stainless‐steel hemispheres cooled with liquid nitrogen. The scavenging products were separated and identified by GC/MS. The condensing efficiencies of the scavenger DMDS and condensable substances from the reaction system were so determined as to make the scavenging R+CH3SSCH3RSCH3+CH3S a quantitative method of radical analysis. It is shown that with a sufficient excess of DMDS the scavenging reaction takes place to 90‐100% for hydrocarbon radicals with two or more C atoms, with a slight interference from addition reaction of radicals to condensable unsaturated products from the reaction system. The type of reactions with mono‐ and divalent radicals and carbenes in the solid DMDS matrix are investigated. Consecutive reactions during warming‐up are also studied and discussed. It is stated that this scavenging method is well suited for the analysis of all hydrocarbon radicals heavier than ethyl, provided that the scavenging products can be separated and identified, which might become a problem with heavy polycyclic aromatic radicals.

The new technique of free‐jet condensation/scavenging with dimethyl‐disulfide has been applied for the quantitative analysis of hydrocarbon radicals and carbenes in flat premixed hydrocarbon/oxygen flames burning with fuel‐rich mixtures at 27 mbar. The results are reported as profiles of mole fractions of the radicals. The limit of detectability was about 1·10−7 mole fraction. Qualitatively the radicals are very similar with the four fuels, but there are large differences in their quantities. While in the methane flame only C1 and C2 radicals were present in detectable concentration, higher radicals up to naphthyl could be detected with the unsaturated fuels. Although methyl could not be determined quantitatively, it was found to be the major hydrocarbon radical in all flames. Methoxy radical was only found in the methane flame. Whereas, for example, C2, C3H2 and C3H are typical high‐temperature species, C2H is surprisingly formed at the beginning of the oxidation zone at relatively low temperature. Vinyl and the vinyl‐type radicals C2nH3 (n = 1 to 4) are in equilibrium with acetylene and the polyynes (C2nH2) at maximum flame temperature and in the burned gas. Since phenyl peaks always after benzene, it is concluded that it is mainly a degradation product from benzene and other lower aromatics. The mechanism of formation of C2H and C2, the equilibria of the C2nH3 and the relation between aliphatic and aromatic radicals are discussed.